077 Combinations

077 Combinations

这道题纯dfs， 但是在 Line 13处做了个剪枝 从84ms 提高到68 ms

 1 class Solution:
 2     def __init__(self):
 3         self.ans = []
 4
 5     def combine(self, n, k):
 6         self.help(n, k, 0, [])
 7         return self.ans
 8
 9     def help(self, n, k, l, cur):
10         if k == 0:
11             self.ans.append(cur[:])
12             return
13         if n - l < k:
14             return
15         for i in range(l, n):
16             self.help(n,k-1,i+1,cur+[i+1])